Anaerobic dechlorination of chlorobiphenyls reduces potential public health and environmental hazards by lowering the degree of chlorination and yielding congeners which are generally less toxic and can readily be degraded by aerobic or anaerobic microbial mechanisms. Reductive dechlorination is advantageous, since it can overcome the difficult problem of degrading or detoxifying highly chlorinated congeners. The activity and pattern of dechlorination are site-specific. PCB-contaminated sediments at the Massena Superfund site showed dechlorination in the laboratory incubation studies and sediment analysis. Our investigation strongly indicated that PCB congeners may also undergo biodegradation in anaerobic sediments. The general goal of this study is to design environmentally- sound and cost-effective bioremediation strategies by determining ecological factors which are essential for the microbial detoxification of PCBs for the Superfund site and similarly contaminated areas elsewhere. The specific aims are (1) to determine the pattern and the extent of biotransformations (reductive dechlorination and biodegradation) in PCB- contaminated sediments in the St. Lawrence River at the Superfund site and measure in situ transformation rates covering a 6-year period, employing the Cs-137 dating technique, (2) to determine the factors regulating biotransformations, especially the effects of polynuclear aromatic hydrocarbons (PAHs) which occur at significant concentrations with PCBs in river sediments receiving discharges from the Reynolds Metals and environmental factors such as organic matter, inorganic nutrients, electron acceptors, trace nutrients, heavy metals, microbial flora, congener composition, sediment texture, microbial populations, etc. The factors will be studied using two approaches: enrichment experiments with the sediments from the study sites and a comparative study of St. Lawrence River and Hudson River sediments, and (3) to confirm that PCB congeners undergo biodegradation and that those with omicron-substitution on only one ring are more readily biodegraded than those with the substitution on both rings. this study will be carried out with three congeners for each substitution type in PCB-free sediments spiked with appropriate congeners with a rigorous material balance and product determination. Biotransformation products will be determined by GC with capillary columns (HP Ultra II and C-87 Apolane), GC with MSD or Fourier transfer infrared detector, HPLC, and thin-layer chromatography.